Mechanical modulator



1953 R. A. HAMPSHIRE ETAL 2,660,709

MECHANICAL MODULATOR Filed Nov. 10, 1951 CARR/E2 506/202- {2 SOURCE 29' 2 8 FIG 2 12m In! /6 5!; I I;

\NVENTORS R A HA MPJHJRE .FW DEM I? HANCOCK ATTORN EY Patented Nov. 24, 1953 UNITED STATES TENT OFFICE MECHANICAL MODULATOR Application November 10, 1951, Serial No. 255,766 3 Claims. (01. 332-56) This invention relates to modulators and more particularly to modulators generally of the mechanical type used for course identifying beacons signals.

Various types of modulators have been provided particularly for applying course identifying signals to radio beacons. In the more usual types of modulators of the known art, the mechanical variety, wherein a motor driven tuning element is used to vary the tuning of networks coupled to transmission lines, has most generally been provided. The network may take the form of a stub connected line or a coupled section of transmission line which in one tuning condition serves substantially to block transmission along the line and in another tuning condition permits energy to be fed through the line to the load, most generally radiating antennas. Instead of utilizing mechanical tuning means switching tubes have been used in such coupled networks for the purpose of providing the desired modulations. However, most of these circuits are not readily adaptable for use with coaxial feed lines and considerable complication in mechanical structure results when coaxial feed is used. The coaxial feed line however provides for better shielding of the feed lines and, therefore, is desirable in many cases.

It is an object of this invention to provide a novel modulator, preferably of the mechanical tuning type, suitable for use with coaxial feeders for supplying energy to radiating loads.

According to the broader aspects of this invention the modulator comprises a two-conductor feed line for coupling a carrier frequency source 1 to a load, the line having provided in one of its conductors a reactor of a given sign, for example an inductor, the other conductor being provided with an arrangement normally providing a high series impedance which may be effectively an open circuit therein. An arrangement is provided for periodically shunting across this high impedance a reactor substantially equal to the given reactance but of opposite sign to provide aseries resonance in the feed line so that energy may be periodically transmitted to the antenna at the modulating rate.

According to a feature of this invention the modulator structure may comprise a coaxial line arranged in the form of a substantially U-shaped loop, one of the conductors, preferably the outer conductor, being cut away to provide a gap therein about the bend in the U. A connection is provided between the cut away conductor portions on the legs of the U at points spaced from the gap, preferably at such points that the sections of transmission line together with this interconnection connecting means provide effectively a balanced quarter wave length stub line presenting substantially infinite impedance to transmission of energy along this conductor. The other conductor, preferably the inner conductor, extends beyond the gap to form the bend in the U and provides a series inductance in this conductor of the line. Connected across the end of the conductor portions at the gap is provided a variable reactor having a capacitive value when completely inserted such that a resultant reactance substantially equal to the inductive value of the inductance of the loop in the inner conductor is provided. Thus as the reactor is varied between its fixed limits the transmission line varies in impedance between substantially infiinite impedance and a series resonant condition at a rate determined by the rate of variation in the reactor. Preferably the reactor is a rotatable condenser unit driven by a motor so that effectively a sine wave variation may be produced in the radiating load coupled to the modulator.

The above mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an em bodiment of the invention taken in conjunction with the accompanying drawings, in which Fig. 1 is a diagrammatic illustration of a glide path radio beacon incorporating modulatorsv in accordance with this invention.

Fig. 1A is an equivalent circuit diagram of the modulator as illustrated in Fig. 1, and

Fig. 2 is an illustration, partly broken away, of structural embodiment of a modulator in accordance with this invention.

Turning first to Fig. 1 there is illustrated the essential circuit diagram of a slide. path radio beacon which has incorporated therein modulators constructed in accordance with this in.- vention. Energy from a carrier frequency source I is fed over a coaxial line section 2 to a modulator bridge arrangement 3 comprising four co.- axial line sections 4, 5, 5 and 1 having a characteristic impedance of times the generator impedance. Each of arms 4;. 5. and 6 arepreferably made one quarter wavelength. long at the carrier frequency while arm 1. is made threequarters of a wavelength long so that the carrier frequency energy will substantially cancelv out at the junction of arms 6 and 1. At this juncture there may be provided a non-dissipative network 8 which can cancel reactive energy when the loads on the bridge are not exactly balanced. In actual operation complete balance will not at all times be present so this compensating network is effective for a mean condition of unbalance. From juncture of arms 4 and T is extended a coaxial feed line 9, folded in the form of a loop, for supplying energy to a first radiator ID. A similar coaxial conductor I I extends from the juncture of arms 5 and 6 to supply energy to a second radiator I2. In practice antennas I and I2 are mounted at different heights above the earth so as to produce overlapping radiating patterns, whi h patterns are to be modulated with diiferent identifying signals so that the course line may be identified on a craft. In order properly to adjust the relative amplitudes of the energy supplied to the two antennas a variable network I3 is shown coupled to the feed line of antenna I2. This network may be adjusted to control the relative amplitude of energy reaching antenna I2.

In order to provide the desired modulation of the energy radiated from the antenna separate modulating networks Id and I are provided in the feed lines 9 and II. The modulation network Hl comprises the substantially U-shaped portion of transmission line 9 having two leg portions I6 and I I and a loop or bend portion I8. The outer conductors of portions IE and I? are cut away at one end to provide a gap 19 about the bend portion I8, a short circuiting bar 20 is provided to interconnect leg portions I6 and IT at a point spaced from the gap It in the outer conductor. Bridged between the outer eonductors I6 and I7 at the gap is provided a variable reactor H which may comprise a pair of stator plates 22, 23 and a rotor plate 2 5. The rotor plate 24 may be in the form of a multiple bladed wheel or disc which is driven at predetermined rate by means of a motor 25 so that the blades are intermittently inserted between the stator plates 22 and 23. The modulator I5 associated with transmission line II is similar in all respects to modulator M except that the tuning reactor 26 thereof is provided with a rotor having a different number of tuning blades so that when it is driven by motor 25 in synchrcnism with rotor 2d a different frequency, to variation in reactor tuning, will be produced. Thus modulator M may produce the 90 cycle signal generally used in radio beacon work, while modu lator I5 produces 150 cycles.

The operation of the modulator to produce the modulation signals may be more clearly understood from the explanation which follows. Preferably the transmission line network is designed to present the desired efiective impedance to the carrier source to match the impedance thereof. The line 2 is of such a length that the carrier source I may be considered effectively to be located at the juncture of arms 4 and I and at the juncture of lines 5 and 6. The portions of lines 9 and II between these respective junctures, and the respective gaps should be an integral multiple of one-half wavelength at the operating frequency so that effectively the generator source may be considered as being located at the gap. Since the outer conductor is severed at this point there will be presented to the source a relatively high impedance. The inner conductor forms a loop I8 at this point so across the gap in the outer conductors there exists a voltage difference, which voltage difierence will tend 4 to produce in legs I6 and IT a transmission of a balanced line nature. The shorting bar 20 if located at a quarter wavelength from the gap will render the portions of the outer conductors in the form of the quarter wave section of balanced transmission line. Under these conditions a high impedance, substantially infinite, will exist between the outer conductors at the gap. Thus there is a discontinuity in the feed line to the antenna which substantially interrupts transmission thereto. This is the condition of minimum energy transfer in the modulation cycle.

The loop I8 formed by the center conductor represents a certain value of inductive reactance. If a capacitive reactance is placed between the outer conductors at the gap which permits a resultant reactance of equal value to loop I3, a series resonant condition will be produced in the feed line resulting in a low impedance across the gap. This will restore continuity in the conductors so that power is transmitted to the antenna. Thus by periodically introducing and removing such a reactance, modulation of energy supplied to the antenna can be produced. Since the voltages on the outer conductors at the gap are of equal amplitude and opposite polarity it will be apparent that in the plane mid-way between the outer conductors there exists an equi-potential region. A conductor can, therefore, be introduced along this equi-potential plane without having any appreciable voltage induced in it. Thus the multi-bladed rotor may be inserted at this point of equi-potential for varying the capacity between these points. It will be clear, therefore, that by rotating such multi-bladed rotor at a desired speed a modulation of the required frequency can be produced. The shaping of the stator and rotor blades is made so as to produce effectively sine wave modulation with minimum distortion.

In the modulator iii the rotor may be provided with live blades while that in modulator id has three blades so that the rotation of these two blades by the motor 25 will produce the desired and 150 cycle modulation. Preferably the rotor and stator blades for the 150 cycle modulator are made smaller than those of the 90 cycle modulator. This is done so as to provide less energy at antenna I2 than at It to achieve the desired glide path energy distribution. An additional control may be effected by network I3.

It will be realized that there is a certain minimum capacity between the stators in reactors 2| and 26 so that corresponding shorting bars are not spaced exactly at a quarter wavelength but are varied from that to take care of this capacitive end efiect. If less than modulation is desired the corresponding shorting bars may be moved so as to make the effective stub sections longer than a quarter Wavelength so that the impedance at the minimum transfer position is reduced permitting some energy to reach the antennas during these points of the modulation cycle. In this way the adjustment of the shorting bars can be used to control the percentage modulation of the tone frequencies.

In the simplified equivalent circuit diagram of Fig. 1A the fundamental operation of the system can be easily seen. In this figure the source I is shown supplying energy to the two conductors 2! and 28 of a two-conductor feed line interconnecting source I with a load 29. In one of these conductors is provided an inductor 3i] and in the other of the conductors is provided a gap 3i bridged by a shorted stub 32. Across the gap 3| is provided a capacitive reactor 33 which may be adjusted to provide series resonance in the line in the manner described above. It will be apparent that while in the preferred form of the invetnion the gap is produced in the outer of the conductors, the system will operate if the gap is provided in the inner conductor together with the variable reactor there-across for compensating the inductance produced by the outer conductor.

In Fig. 2 is shown an actual structural arrangement for the modulators I 4 and i5 shown diagrammatically in Fig. 1. In this figure a box or trough 34 is provided within which the legs l6, I! of the modulator U-loop are mounted. These conductors may be suppotred by insulating supports 36 and S7. A pair of metallic elements 33, 39 are clamped by means of a screw 6% to the outer conductors of legs i5 and it to provide the eifective shorting bar IS. The inner conductors M, 42 are interconnected at their ends by a conductor block :3 to produce the U bend it. At the gap ends of conductor legs l6 and H are provided mounting clamps it and 45 which serve to support the separate stator plates 22 and 23. The multi-bladed rotor 24 is shown mounted in a box portion 36 of a cover plate t! so that the blades may be rotated in the central plane between stators 22 and 23, by means or drive shaft 48.

While this invention has been described specifically with reference to a glide path beacon modulator circuit it will be clear that the modulators may be used with any type of system desired. They may, for example, be used to produce '1 monitoring modulations or may be used for localiser or other types of beacons as well. Likewise, the specific structural arrangement may be readily modified as will be apparent to those skilled in the art.

While we have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. A modulator structure comprising a coaxial transmission line arranged in the form of a substantially U-shaped loop, the outer conductor of said line being cut away to provide a gap therein around the bend in said U, the inner conductor forming a U-shaped loop outside said gap, means interconnecting the conductor portions of said outer conductor in the legs of said U at points spaced from said gap, a variable capacitor coupled between said portions of said outer conductor substantially at said gap, and terminal connections at the ends of said legs for connecting a carrier frequency supplied and a load, respectively thereto.

2. A modulator structure according to claim 1, wherein said means interconnecting said conductor portions is electrically spaced substantially a quarter wavelength at said carrier frequency from said gap.

3. A modulator comprising two coaxial line sections each having inner and outer conductors and being positioned parallel to one another and with one of their ends in alignment, the outer conductors being removed for substantially equal distances adjacent said one end, means for connecting together the inner conductors of said lines at said aligned ends, whereby an effective series inductive impedance is provided in said line, a conductive connection interconnecting the outer conductor of said line sections to provide effectively a quarter wavelength balanced line stub between the point of said interconection and the ends of said outer conductor to produce efiective high series impedance in said line and means for periodically introducing a capacitive reactance between the outer conductors at said one end to produce a series tuning of said line whereby carrier energy supplied to one of said coaxial line sections will be periodicaly modulated at the output of the other of said line sections.

ROBERT ARMITAGE HAMPSHIRE. FLOYD WILLIAM IDEN. PETER L. HANCOCK.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,244,756 Alford June 10, 1941 2,368,693 Watts, Jr. Feb. 6, 1945 2,434,917 Fuchs Jan. 27, 1946 FOREIGN PATENTS Number Country Date 121,502 Australia Aug. 1, 1946 

